This Monday, April 15, 2013 electron microscope image provided by the Centers for Disease Control and Prevention shows the H7N9 virus which can take on a variety of shapes. Almost three weeks after China reported finding a new strain of bird flu in humans, experts are still stumped by how people are becoming infected when many appear to have had no recent contact with live fowl and the virus isn't supposed to pass from person to person. Understanding how the H7N9 bird flu virus is spreading is a goal of international and Chinese experts assembled by the World Health Organization as they begin a weeklong investigation Friday, April 18, 2013. (AP Photo/CDC, C.S. Goldsmith, T. Rowe)

Photo: C.S. Goldsmith, T. Rowe, Associated Press

This Monday, April 15, 2013 electron microscope image provided by...

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A 7-year-old girl, who was confirmed as Beijing's first H7N9 bird flu case, is discharged from Beijing Ditan Hospital. China has reported 87 H7N9 bird flu cases, with 17 deaths.

Photo: Lintao Zhang, Getty Images

A 7-year-old girl, who was confirmed as Beijing's first H7N9 bird...

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This picture taken on April 16, 2013 shows a seller sleeping next to a cage in a poultry market in Guiyang, southwest China's Guizhou province as poultry industry got greatly influenced by H7N9 bird flu. A seven-year-old girl who contracted the H7N9 strain of bird flu left hospital on April 18 and appeared before media in an apparent bid by health authorities to cool concerns about the deadly virus. CHINA OUT AFP PHOTOSTR/AFP/Getty Images

A bird flu that has never before been a problem for humans has infected more than 80 people in China, killing 17 of them, and is raising concerns among infectious disease experts worldwide.

The first human case was identified three weeks ago, and the rapid compilation of human cases since then has public health officials in China and scientists from around the world scrambling to identify the source of the infection and prevent further spread.

So far, the strain identified as H7N9 does not appear to pass easily between humans. Human-to-human transmission is a critical sign that a virus could reach pandemic levels, such as what occurred with the H1N1 "swine flu" that quickly spread around the world in 2009.

It's not clear where people have acquired the H7N9 strain. Exposure to infected birds is suspected because the strain is known to circulate among them, but Chinese public health authorities have not been able to identify a specific source. Meanwhile, several cases of possible human-to-human transmission are under investigation, according to the World Health Organization.

"This has very severe potential to be a pandemic in the nature of H1N1, except now we're talking about a much more virulent virus," said Dr. Charles Chiu, head of the viral diagnostics laboratory at UCSF. "The one caveat is that we still have not documented human-to-human transmission. While that's the case, there's decreased likelihood that it will blow up to a pandemic."

Pandemic concerns

Public health authorities have worried about the possibility of a deadly pandemic flu for decades, and more urgent alarms were sounded in the late 1990s and again in 2003 when the H5N1 bird flu infected people throughout Asia. That potentially deadly strain of flu so far hasn't been able to spread easily among people.

In 2009, the so-called swine flu reached pandemic levels, but it proved to be not much more serious than a typical influenza infection in humans. In the past few years, that virus has become a seasonal fixture and preventable by vaccine.

The most recent strain appears to be very aggressive in humans, causing serious illness and death. What's causing concern among infectious disease experts is that, while it's unclear that the strain is transmissible between humans, the virus has afflicted far more people in just three weeks than H5N1 ever did in such a short time.

'Very worrisome'

"Whereas H5N1 is associated with sporadic, rare cases, this one is associated with many more cases in a very short period of time," Chiu said. "That by itself is very worrisome."

As of Thursday, 87 cases of H7N9 had been confirmed in China, found in four provinces spread around the country, according to the World Health Organization. The first case was identified March 31, but some of those 87 became sick before then.

The strain causes typical flu symptoms - fever, cough, fatigue and muscle aches - but it progresses to pneumonia in many patients, which is not common in most healthy people who become afflicted with influenza.

Almost all of the cases have been serious and required hospitalization. Infectious disease experts in China have found one case of a boy who was infected with the strain but showed no symptoms, meaning the virus is probably more widespread than the lab-confirmed case counts demonstrate.

One possible bit of good news is that an asymptomatic case may mean the virus isn't quite as deadly as infectious disease experts feared. Without knowing the total number of people infected in the population at large, it's impossible for public health authorities to calculate a fatality rate.

"Now that the number of infected people is growing, and the fatality rate is dropping, it's not looking as severe as it once was," said Dr. Cornelia Dekker, an infectious disease expert at Stanford and medical director of the Lucile Packard Children's Hospital Vaccine Program. "That said, we're still very early into this outbreak, and it remains to be seen what we're going to learn."

Aggressive screening

Chinese officials have been aggressively screening people who came into contact with patients who became sick. That screening has identified at least two families with multiple people who were sick, although it's not clear whether the disease spread from one family member to another or if the people became sick from the same source, such as an infected bird.

So far the virus hasn't been identified outside of China. U.S. doctors will pay particular attention to patients who come to them with flu symptoms and have recently traveled from China, said Dr. David Lewis, a pediatrician and infectious disease expert at Stanford.

"At this point there's no need for alarm within the United States," Lewis said. "Over on our side, we'll need vigilance for people who develop respiratory symptoms within 10 days of coming back from China. But that's about all we can do."

Preparing for a vaccine

The Chinese government has offered samples of the new strain to laboratories around the world, in part to start making preparations for developing a vaccine.

The H7N9 strain is carried in birds but does not typically sicken them. That's a problem for public health officials who, in cases of other bird flus, will cull flocks that are sick and dying, thus preventing spread to humans. The inability to identify the source of the H7N9 infection creates a greater challenge for officials to stop spread of the disease.

"What they're hoping to do is put a lid on it by just essentially shutting down the live domestic bird markets. Hopefully that will result in a slowing of spread," Lewis said. "If (H7N9) is not already in another vector like migratory birds, and if the transmission rate is low - not zero, but low - then that would be reason to be optimistic."